Depressed potential operation of electron beam tubes



Ju y 3, 63 J. R. ASHLEY ETAL 3,098,979-

DEPRESSED POTENTIAL OPERATION OF ELECTRON BEAM TUBES Filed Aug. 25, 1961 ENVELOPE DETECTOR R-F OUT COLLECTOR ODULATOR R-F INPUT PIC-3.3.

ENVELOPE Z1 DETECTOR SWITCH TUBE 30 DRIVER 1 To D| GATE- o-x ENVELOPE x 1 39 DETECTOR 20 I40 it i GATE 2 x x INVENTORS 33 JAMES 5. ASHLEY 31 X2 41 YCA/POL M. VERO/VDA f PIC-1.4.

3,098,979 DEPRESSED POTENTIAL OPERATIQN F ELECTRON BEAM TUBES James Robert Ashley and Carol McCauley Veronda, Gainesville, Fla, assiguors to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Aug. 23, 1961, Ser. No. 133,463

' Claims. (Cl. 330-44) This invention relates to the operation of velocity modulation electron tubes with depressed collector potentials, and more particularly relates to the linear operation of such tubes when the tubes are employed in communication system as linear amplifiers for amplitude modulated waves.

In an electron beam velocity modulation device such as a traveling wave tube or a klystron, for example, it has been a common practice to electrically bias the collector electrode at or very near the potential of the microwave structure of the tube. The R.F. properties of the device require that the microwave structure be at a high potential with respect to the cathode of the tube. This in turn requires that the collector also be at a high potential with the result that the electrons in the beam have considerable energy when they strike the collector. This energy is converted largely into heat in the collector and causes considerable power loss and low efiiciency of operation of the tube. In high power tubes additional cooling means must be provided to cool the collector. This cooling equipment is often expensive, bulky, and requires a considerable amount of power to operate. A further disadvantage of operating the collector at a potential near the potential of the microwave structure is that the high velocity electrons striking the collector generate X-rays and give rise to a shielding requirement.

Because of the above-noted problems, considerable effort has been devoted to attempts to operate electron beam velocity modulation tubes such as traveling wave tubes and klystrons with a depressed potential on the collector, i.e., the collector potential is considerably lower than the potential of the R.F. structure. Although the above-mentioned difiicult-ies are largely solved by lowering the potential on the collector, a new problem is created because the collector potential no longer is high enough for the collector to attract all of the low velocity electrons in the beam and the secondary electrons which are produced when the beam strikes the collector. These electrons are drawn to the higher potential R.F. structure and cause noise, non-linearity and instability in the operation of the tube.

The R.F. input-output performance curve of a velocity modulation klystron tube has a shape which is similar to that of a J (X) Bessel function curve (i.e., somewhat similar to a sine curve in the region 0-11). When operating the tube as a linear amplifier in a communication system it is required that the tube operate on the approximately linear leading edge of the curve in order to avoid distortions in the output therefrom. Deviation from linear operation in a klystron tube is caused by reflected beam electrons and secondary electrons traversing the tube in the backward direction and being collected in the electron beam-R.F. wave interaction portion of the tube. A very small backward beam current can cause serious regenerative or degenerative efiects depending upon the phase of the backward traveling bunches of electrons in the beam. It therefore is desired that this backward beam current be as small as possible. This is difiicult to achieve md to maintain with a depressed potential collector when the R.F. input is applied, and is increasingly diflicult to maintain in the presence of an R.F. input whose magnitude continually varies as it would in a com- 3,098,979 Patented July 23, 1963 munications system wherein the input is an amplitude modulated carrier wave.

The difiiculty which has been encountered in the past results from the fact that when the collector is depressed to a fixed voltage in order to achieve optimum efliciency for signals of relatively low magnitude, this collector potential is not sufiicient to attract the low velocity electrons in the beam and the secondary electrons produced when the input signal is of greater magnitude and the velocity spread between slower moving and faster moving electrons is greater. It is for this reason that depressed collector operation of velocity modulated tubes has not been completely successful and has resulted in nonlinearity in the output of the tube when the tube is operated in communications systems as a linear amplifier for amplitude modulated waves.

It therefore is an object of this invention to overcome the above-mentioned difiiculty in the operation of velocity modulation tubes.

It is another object of this invention to operate a ve locity modulation tube with a depressed collector potential when the tube is used as a linear amplifier for amplifying amplitude modulated waves.

A further object of this invention is to obtain optimum efliciency in the operation of a depressed collector velocity modulation tube when the input thereto is com prised of amplitude modulated waves.

These and other objects and advantages of the invention, which will become more apparent from the specification and claims below, are achieved by providing means for applying a depressed potential to the collector of the velocity modulation tube, and means for detecting the envelope of the amplitude modulated input waves to said tube. The envelope of the detected input wave then is combined with the depressed potential biasing voltage applied to the collector so as to increase the potential on the collector in the presence of stronger input signals and to lower the collector potential in the presence of lower amplitude input signals. In this manner, the collector potential is raised to collect the electrons with greater velocity spread produced in response to the higher amplitude input signals, and lowered to collect the electrons when there is very little velocity spread. The collector potential, throughout its varying amplitude, is sufli cient to maintain the operation of the tube in the linear region of its characteristic curve, and yet maintain optimum efliciency, i.e. lowest collector potential possible, in view of the changing amplitude of the input wave.

The invention will be described by referring to the accompanying drawings wherein:

FIG. 1 is a schematic illustration of one embodiment of this invention;

FIG. 2 is a schematic illustration 0st another embodiment of the present invention;

FIG. 3 is a graph used in explaining the operation of the embodiment of the invention illustrated in FIG. 2; and

FIG. 4 is a simplified schematic diagram of the switch tube driver illustrated in block form in FIG. 2.

Referring now more particularly to FIG. 1, the velocity modulation tube 10 is represented in a highly schematic form and is comprised of electron emitting cathode 1-1 and collector electrode 12. The electron lbeam-electromagnetic wav-e interaction structure 15 may be comprised of a number of resonant cavities in the event that tube 10 is a klystron amplifier, or may be comprised of a slow wave propagating structure in the event that said tube is a traveling wave tube. R.-F. input waves are coupled into the tube on input line 16 and are coupled therefrom on output line 17. In practice, lines 16 and 17 may be coaxial or uniconductor waveguide transmission lines. Collector electrode 12 is electrically :biased at a depressed fin-ll potential V of the biasing voltage source.

potential, ie a potential that is lower than the potential of the electron beam-microwave interaction structure 15, as may be seen drom the respective connections to voltage source V and ground as illustrated in FIG. 1. The R.-F. input wave is coupled into tube on line 16, is sampled at point by means of a probe or directional coupler, is detected in envelope detector 21 and the amplitude modulation envelope is applied through collector modulator 22, which may be a power amplifier, and to primary winding 25 of transformer 26. The secondary winding of said tranformer is series connected in the biasing circuit between collector 12 and voltage source V. The wndings of transformer 26 are poled so that a variation of the voltage across primary Winding 25 will be coupled through secondary winding 27 to vary the potential on collector electrode 12 in a corresponding manner.

In the operation of the embodiment of this invention illustrated in FIG. 1, collector 12 is biased at a depressed potential that is some fraction or: voltage source V. This potential is sufiicient to permit efiicient, linear operation of the tube with an input signal of moderate amplitude. An amplitude modulated input wave will be coupled into tube 10 on input line 16 and will cause velocity modulation of the electron beam in the tube. The input waves are coupled from input line 16 at point 20, are detected in detector 21 and coupled through transformer 26 to collector 12 so that the potential on said collector Will be amplitude modulated to follow the amplitude modulation of the input signal. In this manner, when the electrons in the beam are propagated with a relative high velocity spread as a result of a relatively high magnitude input signal, the collector potential also is at a relatively high potential so that as to substantially completely collect the beam electrons and substantially eliminate backward flowing beam electrons. When the magnitude of the input signal is low, the voltage coupled through transformer 26 to collector 12 will lower the potential on collector 12 and permit substantially complete collection of the electrons moving with little velocity spread. 'Ihus efiicient and linear operation of the tube is established and maintained irrespective of changes in amplitude in the input signal.

An alternative embodiment of the present invention is illustrated in FIG. 2 wherein velocity modulation tube .10, its input and output lines 16 and 17 and envelope detector 21 are substantially identical to the corresponding components illutrated in FIG. 1. In FIG. 2, however,

the potential applied to collector 12 may be either one of the potentials V V or V depending upon which of the switch tubes T T or T is rendered conductive by switch tube driver 30. Switch tube driver 30 is a circuit which is responsive to the amplitude of the detected signal from envelope detector 21 and, for example, when the amplitude of the detected signal is within the region o-x on the curve of FIG. 3, only tube T is rendered conductive and the potential V is applied to collector 12. When the magnitude of the input wave is within the region x -x on the curve of FIG. 3, switch tube driver 30 will cause only triode T to conduct so that the potential V is applied to collector 12. Similarly, when the magnitude of the input signal is within the region x -x on FIG. 3, switch tube driver 30 will render only triode T conductive so that the potential on collector 12 is the In this manner, switch tube driver 30 and switch tubes T -T will choose a depressed collector potential which will allow the tube to operate in the linear portion of its characteristic curve while at the same time permitting optimum depression of collector potential in order to achieve optimum efiiciency of operation consistant with desired linearity.

A schematic illustration of the switch tube driver 30 of FIG. 2 is shown in FIG. 4 of the accompanying drawings, and is comprised of input line 31 from envelope detector 21 which is coupled in parallel to diodes 32., 33 and 34. Said diodes are respectively back-biased by the potentials 0 volts, x volts and x volts, these potentials being those illustrated on the curve of FIG. 3. When the magnitude of the input wave is in the region 0-x on the curve of FIG. 3, only diode 32 will be conductive because the back-biasing potentials x and x on the remaining diodes 33 and 34 will prevent those diodes from conducting. When the magnitude of the input wave is in the region x x on the curve of FIG. 3, diode 33 will be conductive, and similarly when the magnitude of the input wave is within the region x x on the curve of FIG. 3, diode 34 will overcome its back-biasing and will conduct. Exclusive energization of only one of the output lines #1, #2 or #3 is provided by means of normally open gates 38 and 39. When diode 33 is rendered conductive the potential coupled to gate 38 on lead 40 will be sufiicient to close the normally opened gate 38. Similarly when diode 34 is rendered conductive the potential applied to gate 39 on lead 41 and will be suflicient to close normally opened gate 39. In this way exclusive energization of only one of the output lines #1#3 is provided.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limi tation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

l. The method of achieving linear operation of a depressed collector velocity modulation electron tube when used as a linear amplifier for amplitude modulated waves comprising the steps of abstracting a sample of the input wave to be amplified, passing the sampled wave through a detector to obtain the envelope of the amplitude modulation of said input wave, and controlling the potential on the collector of said tube in accordance with the amplitude of said envelope to increase the potential on the collector when a high amplitude signal is coupled to the input of said tube and decrease the potential on said collector when a lower amplitude signal is coupled to the input of said tube.

2. Means for operating a velocity modulation electron tube with a depressed potential on its collector electrode and for obtaining linear operation of said tube when said tube 'is used as an amplifier :for amplitude modulated waves comprising the combination of means for applying a biasing voltage to the collector electrode of said tube, said voltage being below the voltage of the electron beamelectromagnetic wave interaction structure of said tube, means for coupling amplitude modulated waves to be amplified into said tube, means for deriving a signal representing the amplitude modulation envelope of said input waves, means for applying said envelope signal to the collector electrode to increase the potential on said collector when a high amplitude signal is coupled to said tube and to decrease the potential on said collector when a lower amplitude signal is coupled to said tube.

3. Means rfor operating a velocity modulation electron tube with a depressed potential on its collector electrode and for obtaining linear operation of said tube when said tube is used as an amplifier for amplitude modulated waves comprising the combination of means for applying a biasing voltage to the collector electrode of said tube, said voltage being below the voltage of the electron beam-electromagnetic wave interaction structure of said tube, means for coupling amplitude modulated waves to be amplified into said tube, means for deriving a signal representing the amplitude modulation envelope of said input waves, means for controlling the potential applied to said collector electrode in accordance with said envelope signal to increase the potential on the collector when said envelope signal is of a high magnitude and to decrease the potential on said collector when said envelope signal is of a lower magnitude.

4. Apparatus for achieving linear operation of a depressed collector velocity modulation eleotron tu be when said tube is operated as a linear amplifier for amplitude modulated input Waves, comprising detector means for deriving a signal representing the amplitude modulation component of said input waves, a transformer having its primary winding coupled to pass said amplitude modulation signal component and its secondary winding coupled in a voltage biasing circuit for the collector of said tube, said secondary winding being so coupled and arranged to increase and decrease the 'biasing voltage on said collector in accordance with the respective increase and decrease in the magnitude of said amplitude modulation signal component.

5. Apparatus for achieving linear operation of a depressed collector velocity modulation electron tube when said tube is operated as a linear amplifier :Eor amplitude modulated input waves, comprising detector means for deriving a signal representing the envelope of the amplitude modulated input waves, means for providing a plurality of biasing voltage potentials, means for connecting anyone of said plurality of voltage potentials to the collector electrode of said tube, all of said potentials being lower than the potential of the electron beam-electnomagnetic wave interaction structure of the tu be, means responsive to the magnitude of envelope signal to select one of said connecting means for coupling a respective one of said voltage potentials to said collector, said con-necting means and said envelope signal responsive means cooperating to couple a higher potential to said collector when the amplitude of said envelope signal is high and to couple a lower potential to said collector when the amplitude of said envelope signal is lower.

No references cited. 

1. THE METHOD OF ACHIEVING LINEAR OPERTION OF A DEPRESSED COLLECTOR VELOCITY MODULATION ELECTRON TUBE WHEN USED AS A LINEAR AMPLIFIER FOR AMPLITUDE MODULATED WAVES COMPRISING THE STEPS OF ABSTRACTING A SAMPLE OF THE INPUT WAVE TO BE AMPLIFIED, PASSING THE SAMPLE WAVE THROUGH A DETECTOR TO OBTAIN THE ENVELOPE OF THE AMPLITUDE MODULATION OF SAID INPUT WAVE, AND CONTROLLING THE POTENTIAL ON THE COLLECTOR OF SAID TUBE IN ACCORDANCE WITH THE AMPLITUDE OF SAID ENVELOPE OF INCREASE THE POTENTIAL ON THE COLLECTOR WHEN A HIGH AMPLITUDE SIGNAL IS COUPLED TO THE INPUT OF SAID TUBE AND DECREASE THE POTENTIAL ON SAID COLLECTOR WHEN A LOWER AMPLITUDE SIGNAL IS COUPLED TO THE INPUT OF SAID TUBE. 